1. Field of the Invention
This application relates generally to downhole tools, and more particularly to a thermal insulation vessel that may be used in conjunction with downhole tools for thermally isolating various components.
2. Description of the Related Art
Oil and gas wells subject downhole tools to extreme environmental conditions. Ambient pressures can be several orders of magnitude greater than atmospheric pressure. Temperatures can exceed 200.degree. C., and loads and vibrations associated with fluid flow, string weight and impacts with formations and casing can be immense. The design of tools to operate in the downhole environment involves careful consideration of these pressure, temperature and load factors.
Throughout much of the history of the oil and gas well industry, heat transfer considerations played a subordinate role to other design considerations, such as tool static and fatigue strength, seal integrity, and corrosion resistance, to name just a few. With the advent of tools incorporating various electrical components, such as logging tools, measurement while drilling ("MWD") and logging while drilling ("LWD") tools, heat transfer considerations became more important and designers began to turn their attention toward providing thermal insulation for certain types of thermally sensitive electrical and electronic components housed within a tool. There are currently many examples of components used in downhole tools that may benefit from thermal protection. Examples of these include, integrated circuits, sensor packages, battery packs, and electric motors to name just a few.
One type of downhole tool employed in oil and gas wells is an initiating device or initiator. An initiator is commonly used to provide a short burst of high pressure gas or a gaseous mixture that is used to actuate some type of mechanical mechanism in another downhole tool, such as a packer, an intervention tool, or other such tool. Many conventional initiators consist of a tubular housing that encases a firing head which includes a propellant charge for delivering the high pressure gaseous mixture, and an onboard power and control system. The initiator is brought into engagement with the packer or intervention tool either at the surface or downhole, and fired with the aid of a timer set to trigger at a preselected time after downhole insertion or by command sent from the surface. After the initiator fires, it is normally withdrawn from the bore hole. As with many types of modern tools, initiators can incorporate components that may benefit from thermal isolation, such as battery packs and integrated circuits.
Heat transfer between structures within a downhole tool involves a complex combination of conductive, convective and radiative heat transfer. Although, conduction is often the primary heat transfer mechanism, forced convection may be significant where there is through-tool and external fluid flow. Natural convection can come into play where fluids such as air and hydraulic fluids are housed within the tool. Several methods have been employed in the industry to control heat transfer in downhole tools.
Some conventional downhole tools rely upon the forced convective heat transfer associated with mud or other working fluid flow through the tool to carry away heat. Others incorporate heat sinks into the internal structure of the tool. Still others attempt to shield or otherwise isolate a thermally sensitive component from ambient sources of heat. Some of these conventional thermal isolation designs involve the encasement of the thermally sensitive component within a shell or housing that is provided with a thermally insulating blanket or jacket that shrouds the housing. Another common conventional thermal isolation design involves the encasement of the thermally sensitive component within a tubular flask that is, in turn, encased within another housing and supported therein by a plurality of support pegs that are in physical contact with the outer housing and the inner flask. Various materials have been used to fabricate the support pegs, such as carbon and alloy steels, aluminum, and- synthetic materials, such as plastics, and various ceramic materials.
There are several disadvantages associated with conventional thermal isolation designs. Reliance on forced convection via a working fluid introduces unpredictability, as actual flow rates, densities and temperatures observed downhole may deviate from anticipated norms. Those designs which incorporate an insulation flask supported by pluralities of support pegs reduce somewhat the potential for conductive heat transfer between the component in the flask and external structures. However, the pegs themselves still present multiple conductive heat transfer pathways. This is particularly so where the support pegs are fabricated from materials with relatively high thermal high conductivities, such as metallic materials. The incorporation of support pegs fabricated from non-metallic materials with lower thermal conductivities reduces the potential for damaging heat transfer for a given flask. However, even with non-metallic support pegs, there remains a plurality of physical conductive heat transfer pathways. Where the temperature difference between the interior and the exterior of the flask, i.e., .DELTA.T is large enough, significant heat transfer may still occur across the support pegs.
The present invention is directed to overcoming or reducing the effects of the one more of the foregoing disadvantages.